In estimating the amount of carbonated alkali, it is requisite to determine, first, the actual amount of alkali existing in the soda or potash, and this being ascertained, the quantity of carbonated alkali is reduced by calculation. Fifty grains of the alkaline sample are to be dissolved in a flask containing 2 or. of water. Next weigh out, on a watch-glass, 100 grains of well-crystallized oxalic acid, reduced to a fine powder. Small portions of this powder are to be added at a time to the alkaline solution, shaking the liquid between each addition, or stirring it with a glass rod, heating and testing it with litmus paper till the latter becomes slightly reddened, while the liquid is hot. The residue of the oxalic acid is then weighed, and supposing it is 43 grains, it is obvious that to saturate the alkali in the 50 grains of the sample, 57 grains of oxalic acid were consumed; 7.87 grains of oxalic acid are capable of saturating or removing the alkaline reaction of 5 grains of caustic soda, or 7 grains of caustic po-tassa.
These may be soluble or insoluble. As they are not taken up by the ley, the soapmaker need care nothing about the insoluble substances. Generally the soluble ones are found to be chlorides or sulphates. The former are detected by adding a solution of nitrate of silver to a clear solution of the substance to be examined, which has been previously slightly acidulated with chemically pure nitric acid, and if there is chloride of potassium or salt present, a white curdy precipitate will be formed, which, by exposure to light, becomes first violet, and afterwards black. Sulphates are detected by first neutralizing the solution with nitric acid, and then adding a solution of chloride of barium; a fine heavy white precipitate is formed. To many it is of importance to ascertain- if there is any sulphide of sodium, because a potash or soda containing it would be unfit for the manufacture of white soap. It is often detected in the potash and soda, but never in the caustic soda. Its presence will be indicated by the development of hydrosulphuric acid, on adding an acid to a solution of the alkali, a gas very much resembling rotten eggs in its smell.
Where the odour of the gas fails to afford sufficient proof of the presence of hydrosulphuric acid, the application of the following reagent will remove all doubt. The air suspected to contain it is tested by placing in it a small slip of paper, moistened with a solution of acetate of lead; if the gas is present, the slip will be covered with a thin, brownish-black, shining film of sulphide of lead.
Preparation of the Leys. Water. - Only spring or river water should be used in making soap. It must also be perfectly clear, otherwise clear ley cannot be produced. It must be free from organic matters, for these are often dissolved, and, though imperceptible, soon cause the water to become putrid. Nearly all waters contain mineral matters in solution. When such waters are used, though the leys are equally good, there will be a loss of material in proportion to the quantity of alkali neutralized. A water containing more than twelve grains of such substances in one gallon should be rejected.
Ley is an aqueous solution of caustic soda or potassa, by the agency of which the chemical decomposition of the fat and its conversion to soap are effected. Caustic soda is a commercial commodity, but it may happen that the soapmaker will have to prepare his own leys. 1. Reduce the soda or potassa into small pieces, mix it with slacked lime, let it stand 24 hours, and then leach it out with water. For this purpose large tanks are used, having a perforated floor, placed from two to four inches above the bottom, and covered with a layer of straw, on which is poured the mixture of lime with the alkali. A faucet is inserted between this perforated floor and the bottom, by means of which the liquor can be drawn off. The leys prepared in this way are never perfectly caustic; whilst in this process more lime is requisite than when the following method is adopted, which gives a perfectly caustic soda. 2. The potash or soda, not too concentrated a solution, should be thoroughly brought together with lime-milk, this process being assisted by heat. Insoluble carbonate of lime forms, which settles at the bottom.
There should not be more than about 15 per cent. of alkali in the solution, otherwise a portion of the carbonated alkali will remain un-decomposed. For the thorough decomposition of the carbonates of the alkalies, the process of boiling must be continuous and uninterrupted, and the lime of a milky consistency. To ascertain whether the ley is caustic, take a test-glass full, let it stand till cool, then filter, and drop into the clear liquid some nitric acid; if it effervesces, the ley is not caustic; the boiling has to be continued till the portion taken from the kettle shows, when filtered, no escape of carbonic acid if nitric acid be added. As soon as no carbonic acid escapes from the ley, the fire should be taken out, the liquor carefully covered, and suffered to remain undisturbed for 12 or 15 hours, so that the lime may settle. After this, the clear liquor should be transferred by a siphon into a wooden vat, lined inside with sheet lead, and having a perforated false bottom, and cock fitted near the bottom so that the clear ley may be drawn off. The lime used must not have been exposed to the atmosphere; only the quantity actually required should be slacked at a time, because the hydrate of lime, as well as the leys, loses its causticity when exposed to the air.
For 100 lbs. of crystallized soda, 24 lbs. of quick-lime arc required; for 100 lbs. of pearl ash, double that quantity; and for 100 lbs. of soda ash, 60 lbs. will be required. For the transformation of pearlash or soda into caustic leys, more or less quick-lime is necessary, according to the amount of carbonated alkalies they contain, and an excess of lime will do no harm.